294
H . KLOMP
addition one or more delayed density dependent factors have a compensating influence on numbers.
D. DELAYED DENSITY DEPENDENT MORTALITY
1. The Role of Parasitoids
Many years ago it was pointed out by Howard (1897) and Howard
and Fiske (1911) that insect parasites must play a significant part in the
dynamics of the populations of their hosts. These assertions were based
mainly on field observations showing increasing percentages of infected
hosts when the latter approached infestation levels. These statements
later attracted the attention of Nicholson (1933) and Smith (1935). The
former has demonstrated in an arithmetic theory, which was later also
mathematically formulated (Nicholson and Bailey, 1935), how host
and parasite should interact to result in oscillations around fixed levels
of density.
The first condition to be fulfilled is the synchronization of host and
parasite generations. Secondly, according to this theory, when host
density increases the parasite population meets more favourable conditions for oviposition, because of an increasing chance of finding a host.
This phenomenon, resulting in a higher effective fecundity of the parasite, has been referred to afterwards by Solomon (1949) as being the
parasite’s functional response to host density. The result of this response is that notwithstanding the increase of host density the fraction
of hosts infected remains about constant, assuming an invariable parasite density.
The third starting point of the theory is the assumption that the
functional response gives rise to an increase of the numbers of the parasite in the next generation, a phenomenon afterwards indicated by
Solomon as the parasite’s numerical response to host density. According
to the theory, this rise in parasite density results in a higher fraction
of hosts being infected in this new generation, independently of the
density of the host. Consequently, the density dependent reaction expressed as an increased percentage mortality is realized one generation
after the increase in host numbers which initially put the system into
action. Therefore, Varley (1947) has indicated this type of parasite
response as being delayed density dependent.
This theory of Nicholson> has been criticized on several points. One
of the objections levelled against it is the ever-increasing amplitude of
the oscillations (Varley, 1947), which are in fact never observed under
natural conditions. Tinbergen and Klomp (1960) have presented some
models showing that these oscillations can be damped by the introduction of a density dependent factor into the host-parasite system. Varley
and Gradwell (1962) have suggested that in the winter moth, Operophteru
H . KLOMP
addition one or more delayed density dependent factors have a compensating influence on numbers.
D. DELAYED DENSITY DEPENDENT MORTALITY
1. The Role of Parasitoids
Many years ago it was pointed out by Howard (1897) and Howard
and Fiske (1911) that insect parasites must play a significant part in the
dynamics of the populations of their hosts. These assertions were based
mainly on field observations showing increasing percentages of infected
hosts when the latter approached infestation levels. These statements
later attracted the attention of Nicholson (1933) and Smith (1935). The
former has demonstrated in an arithmetic theory, which was later also
mathematically formulated (Nicholson and Bailey, 1935), how host
and parasite should interact to result in oscillations around fixed levels
of density.
The first condition to be fulfilled is the synchronization of host and
parasite generations. Secondly, according to this theory, when host
density increases the parasite population meets more favourable conditions for oviposition, because of an increasing chance of finding a host.
This phenomenon, resulting in a higher effective fecundity of the parasite, has been referred to afterwards by Solomon (1949) as being the
parasite’s functional response to host density. The result of this response is that notwithstanding the increase of host density the fraction
of hosts infected remains about constant, assuming an invariable parasite density.
The third starting point of the theory is the assumption that the
functional response gives rise to an increase of the numbers of the parasite in the next generation, a phenomenon afterwards indicated by
Solomon as the parasite’s numerical response to host density. According
to the theory, this rise in parasite density results in a higher fraction
of hosts being infected in this new generation, independently of the
density of the host. Consequently, the density dependent reaction expressed as an increased percentage mortality is realized one generation
after the increase in host numbers which initially put the system into
action. Therefore, Varley (1947) has indicated this type of parasite
response as being delayed density dependent.
This theory of Nicholson> has been criticized on several points. One
of the objections levelled against it is the ever-increasing amplitude of
the oscillations (Varley, 1947), which are in fact never observed under
natural conditions. Tinbergen and Klomp (1960) have presented some
models showing that these oscillations can be damped by the introduction of a density dependent factor into the host-parasite system. Varley
and Gradwell (1962) have suggested that in the winter moth, Operophteru